This invention relates to apparatus for conducting a continuous chemical reaction with internal recycle and, more particularly, relates to apparatus useful for the esterexchange conversion of ethylene glycol-degraded polyethylene terephthalate.
High molecular weight polyesters of terephthalic acid and aliphatic dihydric alcohols are well known in the art. Polyethylene terephthalate is a commercially preferred polyester of this class due to its exceptional physical and chemical properties.
Polyethylene terephthalate is typically prepared by contacting an organic ester of terephthalic acid, such as dimethyl terephthalate, with ethylene glycol in the presence of an ester exchange catalyst to form dihydroxyethyl terephthalate monomer, and then polymerizing the monomer to high molecular weight using condensation polymerization techniques. Details of this process are disclosed in U.S. Pat. No. 2,465,319 to Whinfield and Dickson. Various inert additives, such as slip additives, are generally added during the process to adapt the polyester for its intended commercial use as a packaging film, fiber, electrical insulator, molded article, etc.
Considerable waste is generated as the polyester is manufactured into commercial form. For instance, edge trim, slitting trim, and reject material is accumulated as polyethylene terephthalate is extruded, biaxially stretched, and slit into film widths desired by customer industries. The industry has proposed a variety of processes for reclaiming these wastes in order to conserve resources and eliminate ecological problems associated with waste disposal.
One proposal has been to (1) degrade the polyester wastes with the glycol used in making the polyester to prepare dihydroxyalkyl/terephthalate, followed by (2) reacting the degraded wastes with a monohydric alcohol to convert the terephthalic acid values to dialkyl terephthalate. The dialkyl terephthalate, when recovered, would be recycled to prepare fresh polyester.
East German Pat. No. 69,500, for instance, discloses a waste recovery procedure wherein (1) polyethylene terephthalate is degraded with ethylene glycol, and (2) the degraded product is reacted with methanol in the presence of an ester exchange catalyst under superatmospheric pressure and at an elevated temperature to prepared solution containing dimethyl terephthalate. After the release of the superatmospheric pressure, the solution is cooled to crystallize dimethyl terephthalate which is then recovered using a centrifuge.
Although the glycol-degradation step disclosed in the German patent is satisfactory, the methanol ester exchange and recovery steps are not readily adapted for continuous commercial operation. Cooling of the solution to crystallize dimethyl terephthalate causes a substantial heat loss since excess methanol contained in the solution must be reheated for recycle. Also, the recovered dimethyl terephthalate contains occluded contaminants which detract from the properties of polyester made therefrom, inert additives present in the wastes, and some of the ester exchange catalyst. Presence of these materials complicates quality control in manufacture of polyester made from recovered dimethyl terephthalate. Moreover crystallization recovery techniques are better adapted to a batch process than the more desirable continuous process.
Thus, there is a need for an improved process and apparatus for preparing dialkyl terephthalate from glycol-degraded polyester wastes. Especially desirable is apparatus which can readily be integrated in a continuous commercial polyester waste recovery operation.